This invention relates generally to disk drive systems. More particularly, this invention relates to a method and apparatus for run-out correction in a disk drive.
Personal computers typically connect to an optical disk drive such as a CD-ROM or DVD-ROM to read data from a compact disk. On the compact disk, data is stored in the form of pits and lands patterned in a radial track. The track is formed in one spiral line extending from the inner radius of the disk to the outer edge. A pit is a location on the disk where data has been recorded by creating a depression in the surface of the disk with respect to the lands. The lands are the areas between the pits in the tangential direction. The reflectivity of the pits is less than the reflectivity of the lands. To store audio or digital information, the length of the pits and lands are controlled according to a predefined encoding format.
When reading information from the disc, light from a laser beam is directed onto the track and the light beam is reflected back to a photo-sensor. Since the pits and lands have different reflectivity, the amount of reflected light changes at the transitions between the pits and the lands. In other words, the encoded pattern of the pits and lands modulates the reflected light beam. The photo-sensor receives the reflected light beam, and outputs a modulated signal, typically referred to as a RF signal, that is proportional to the energy of the light in the reflected light beam.
In FIG. 1, the relationship of the RF signal to the pits and lands is shown. A smaller pit or land decreases both the period and the amplitude of the RF signal. The RF signal in the pits and lands has opposite polarity.
One encoding format used in optical disk systems is eight-to-fourteen modulation (EFM). EFM reduces errors by minimizing the number xe2x80x9czero-to-one one-to-zero transitions. In other words, small pits are avoided. In EFM, the data signal includes no less than two zeros and no more than ten zeros between logical transitions at the pit edges. A zero is indicated by no change in the energy of the reflected beam for at least two clock periods. A one is indicated by a change in the energy of the reflected light beam, that is, a pit edge. Applying the EFM encoding rules, a pit or land will have a length corresponding to the amount of time for at least three and up to thirteen clock periods and the electronics will output a corresponding voltage as shown in FIG. 1.
In an optical disk drive, an optical head assembly includes the photo-sensor, a tracking actuator and a lens. The optical head assembly is mounted on a sled. The tracking actuator is supported by the sled. The lens is not directly attached to the sled, but is coupled to the tracking actuator. The lens is positioned between the photo-sensor and the disk to transmit the light beam from the laser onto the disk surface and to transmit the reflected light beam to the photo-sensor. The sled and tracking actuator position the lens with respect to the spiral track. The sled is driven by a sled motor that positions the head assembly radially across the disk. The tracking actuator is a voice coil motor (VCM) that positions the lens within the limits of the sled. Because the geometry of the photo-sensor is large with respect to a single track, the lens can be positioned within a range of tracks and the photo-sensor can properly detect the RF signal.
A search is performed to position the head assembly and lens over a target region of the spiral track. During searching, track crossings will be detected as the lens is moved radially across the spiral track. The track crossings provide relative position information with respect to an initial position on the disk.
For rough searches, the sled and sled motor provide primary positioning of the head assembly and lens. For fine searches, the tracking actuator provides primary positioning of the lens. A tracking actuator drive signal is used to control the tracking actuator.
For rough searches, some disk drives derive a sled motor drive signal from the number of tracks over which the head assembly is to be moved. A play mode follows each search to read address information and to perform a fine search to position the lens at a desired spiral address. Typically, the play mode is extended to allow the lens position to be re-centered with respect to the sled, thus further increasing access time.
A problem called run-out occurs for several reasons. Run-out occurs when the tracks on the disk are not circular, but oval. Run-out also occurs when the spindle motor does not rotate the disk in a perfectly circular manner but wobbles. In addition, run-out occurs when the center of the disk is not concentrically aligned with the center of the axis of rotation of the spindle motor.
Run-out creates problems during both playing and searching. During play mode, the lens must be moved to follow the eccentricity of the track. In optical disk drives, such as DVD disk drives, the amount of run-out may be equal to 190 track pitches.
During search mode, particularly when the optical disk drive is rotating the disk at a high speed, such as 4,000 revolutions per minute (RPM) or more, the lens encounters a higher speed relative to the disk, creating higher relative motion between the lens and the disk. For instance, when the head assembly and lens are stationary in an absolute fixed radial position with respect to a disk that has a run-out of 190 tracks and the disk is rotating at 4,000 RPM, the head assembly will detect a relative velocity of at least 2,800 tracks per second, even though the lens and head assembly are not moving. Under these operating conditions, when the lens is being moved at an actual absolute velocity of 2,800 tracks per second, the detected relative velocity will range from zero tracks per second to as much as 5,600 tracks per second depending on the position of the lens with respect to the peak run-out. Therefore, without correcting for run-out, it is difficult to accurately search across the disk.
During play mode, the tracking servo is on and the lens follows the spiral track. If the run-out is high, then there will be a residual off track error which can degrade the read signal from the main beam. The magnitude of the off-track error depends on the tracking servo loop gain at the run-out frequency. Typically, the peak run-out and the tracking servo loop gain at the run-out frequency are specified for disks, and if a disk meets the specification, reading is not impaired. Often, the total run-out exceeds that of the specifications while the specifications limit the tracking servo loop gain, and such disks may induce read errors. Therefore, there is a need to improve the readability of disks when the run-out exceeds specifications while maintaining the amount of tracking servo loop gain to within specifications.
In one method of correcting for run-out during play mode, the tracking error signal is band-pass filtered and used as a feed-forward signal to drive the tracking actuator. This method works well in constant angular velocity (CAV) play mode, but is not suitable for those disk drives operating at a constant linear velocity (CLV) such as CD or DVD disks because the rotational frequency, and therefore the run-out frequency, constantly changes.
In view of the foregoing, it would be highly desirable to provide a method and apparatus to correct for run-out in disk drives operating in both a constant angular velocity and a constant linear velocity.
A center error signal is refined with a run-out reducing gain factor which is included in a servo drive signal that is supplied to a tracking actuator. The center error signal represents the relative lens to disk motion. When the track servo is on, as the disk rotates, the center error signal will change and include deviations caused by the tracking servo tracking the run-out. The deviations caused by run-out will be referred to as a run-out component. The invention uses the run-out component to control the tracking actuator.
A method selects a run-out reducing gain factor. A run-out correction signal is produced by applying the run-out reducing gain factor to a center error signal. A servo drive signal is refined with the run-out correction signal to produce an adjusted servo drive signal. A tracking actuator is driven with the adjusted servo drive signal.
In an alternate embodiment, the center error signal is associated with the rotational position of the spindle motor, sampled and stored in a memory synchronized to the rotational position of the spindle motor. The sampled center error signal is retrieved from the memory in synchronization with the rotational position of the spindle motor. A run-out correction signal is produced by applying a run-out reducing gain factor to the retrieved center error signal. A servo drive signal is refined with the run-out correction signal to produce an adjusted servo drive signal. A tracking actuator is driven with the adjusted servo drive signal.
In another aspect of the invention, the center error signal is modified to compensate for the mechanical dynamics of the disk drive.
Alternately, an apparatus corrects for run-out in an optical disk drive. A preamplifier generates a center error signal. A high pass filter generates a filtered center error signal by removing at least a DC component in the center error signal. Another filter produces a filtered servo drive signal by applying a transfer function that compensates for the dynamics of the tracking actuator. A multiplier multiplies the filtered center error signal by a run-out reducing gain factor to produce a run-out correction signal. A summer adds the run-out correction signal to the filtered servo drive signal to produce an adjusted servo drive signal that is supplied to a tracking actuator.
In yet another aspect of the invention, a disk drive uses the apparatus of the present invention.
In this way, using the center error signal, run-out is corrected at all speeds when a disk drive is operated at either a constant angular velocity or a constant linear velocity. In addition, the invention corrects for run-out in both play and search modes. Therefore, the run-out correction will more accurately position the head and reduce access time.